Master defence by Yeghishe Tsaturyan

Ultra-high Q micromechanical resonators for cavity optomechanics

The field of cavity optomechanics has emerged as a strong candidate for future quantum technologies, offering exciting avenues to explore – both in fundamental research and technological applications – ranging from force sensors, microwave-optical transducers, to explorations of quantum states of light and matter at truly macroscopic length scales. The ever-growing ambitions and curiosity of the experimenter inevitably requires perfection of the constituents in an optomechanical system, which, in turn, necessitate a careful analysis of various loss mechanisms.

In this work, I report on progress in understanding and addressing the most prominent sources of mechanical dissipation in our optomechanical system. The system in consideration consists of a tensioned dielectric membrane resonator embedded in a Fabry Peròt cavity. In continuation of our previous work, we develop silicon nitride membranes embedded in two-dimensional silicon phononic crystal structures, showing mechanical Q factors above ~107 at cryogenic temperature at megahertz frequencies for a multitude of modes, indicative of suppressed phonon tunneling losses.

With this experience in hand and a careful analysis of other loss mechanisms, we recognise the importance of eliminating friction losses due to bending in membrane resonators. To this end, we develop a new type of micromechanical resonator, where localisation of vibrations by means of phononic crystal patterning of the membrane itself results in a reduced curvature in the displacement field of the membrane and thus reduced losses due to bending. With these devices we demonstrate a Qf-product of (1.66 ± 0.02)×1014 Hz at room temperature, surpassing all other microfabricated mechanical resonators to date.